Between 1905 and 1907, Einstein first tried to extend the special theory of relativity in such a way so as to explain gravitational phenomena. This was the most natural and simplest path to be taken. These investigations did not fit in with Galileo’s law of free fall. This law, which may also be formulated as the law of the equality of inertial and gravitational mass, was illuminating Einstein, and he suspected that in it must lay the key to a deeper understanding of inertia and gravitation. In 1907 imagined an observer freely falling from the roof of a house; for the observer there is during the fall – at least in his immediate vicinity – no gravitational field. If the observer lets go of any bodies, they remain relative to him, in a state of rest or uniform motion, regardless of their particular chemical and physical nature. The observer is therefore justified in interpreting his state as being “at rest”.x

Already in 1919, Einstein told a New York Times correspondent (who came to his Berlin home to interview him) about the above thought experiment. The correspondent reported about it in the following way, “It was during the development of the formulas for difform motions that the incident of the man falling from the roof gave me the idea that gravitation might be explained by difform motion”. The correspondent transformed the thought experiment from Bern to Berlin and into a realistic amusing story. Presumably Einstein told the correspondent the story in this way, and he did not notice that Einstein was fooling him: “It was from his lofty library, in which this conversation took place, that he observed years ago a man dropping from a neighboring roof – luckily on a pile of soft rubbish – and escaping almost without injury. This man told Dr. Einstein that in falling he experienced no sensation commonly considered as the effect of gravity, which, according to Newton’s theory, would pull him down violently toward the earth”.x

Newton’s apple is also mentioned in this article. Newton must have had something in mind when he compared the moon’s centrifugal force with gravity – one of the hints leading him to the universal law of gravitation – and there is every reason to believe that the fall of an apple gave rise to it. William Stukely wrote that Newton told him the story and Conduitt also reported that Newton told him the same story about the apple; and others also reported that Newton was musing in his mother’s garden in Lincolnshire, there came to him a thought about gravitation upon seeing a falling apple

However, it seems that Einstein was not inspired by Newton’s apple. He was inspired by Galileo’s law of free fall. Newton recognized that this law implied the equality of inertial and gravitational mass

Newton realized that Galileo’s law of free fall is connected with the equality of the inertial and gravitational mass; however, this connection was accidental. Einstein said that Galileo’s law of free fall can be viewed as Newton’s equality between inertial and gravitational mass, but for him the connection was not accidental

Einstein’s 1907 breakthrough was to consider Galileo’s law of free fall as a powerful argument in favor of expanding the principle of relativity to systems moving non-uniformly relative to each other. Einstein realized that he might be able to generalize the principle of relativity when guided by Galileo’s law of free fall; for if one body fell differently from all others in the gravitational field, then with the help of this body an observer in free fall (with all other bodies) could find out that he was falling in a gravitational field